Fresh air flow and exhaust gas recirculation control system and method

ABSTRACT

A fresh air and exhaust gas control method for an engine includes monitoring parameters of an engine in an operational state using a plurality of sensors and generating engine state estimates using an engine observer model. The engine observer model represents an intake manifold volume, an exhaust manifold volume, and a charge air cooler volume. The method also includes generating a turbocharger rotational speed estimate using a turbocharger model and calculating a fresh air flow correction factor. The method further includes determining a desired air throttle position and a desired EGR valve position based on setpoint commands, the monitored engine parameters, the fresh air flow correction factor, the engine state estimates, and the turbocharger rotational speed estimate. The method additionally includes adjusting the air throttle based on the desired air throttle position and adjusting the EGR valve based on the desired EGR valve position.

FIELD OF THE DISCLOSURE

The present disclosure relates to engine control systems and methods andmore particularly to an engine air system model that can run in theengine control unit (ECU) in real time.

BACKGROUND

Internal combustion engines use recirculated exhaust gas to reduceemissions of nitrogen oxides (NOx). Achieving the optimum combination ofair and burnt gasses within the engine cylinder prior to the start ofcombustion on each engine cycle may not be possible due to the dynamicsof the air system. To address this problem, control strategies are usedto attempt to regulate the intake manifold pressure and the percentageof exhaust gas recirculation (EGR). Traditional engine controlstrategies use open loop tables and/or proportional-integral-derivative(PID) controllers. These systems tend to have errors under transientconditions because the open loop tables are calibrated for steady stateconditions and the closed loop PID controls depend on sensor feedback,which inherently has lag. A physics-based feed forward calculation canbe used instead of the open loop tables and PID controls. The feedforward calculation can provide better response under transientconditions, prioritize control objectives, account for systemconstraints, and be easier to calibrate.

It would be desirable to have engine control systems and methods thatregulate the intake manifold pressure and percent EGR usingphysics-based feed forward calculations.

SUMMARY

A fresh air and exhaust gas control method is disclosed for an enginehaving an air system with an aft throttle and an exhaust gasrecirculation (EGR) valve. The control method includes monitoringparameters of the engine using a plurality of sensors; modeling the airsystem and generating engine state estimates using an engine observermodel; generating measured engine states based on the monitored engineparameters from the plurality of sensors; computing an observer errorbased on the differences between the measured engine states and theengine state estimates; generating model corrections; generatingactuator commands and adjusting the air throttle and the EGR valvepositions based on the actuator commands. The model corrections arebased on the observer error, and the model corrections are input to theengine observer model. Generating actuator commands includes generatinga desired air throttle position and a desired EGR valve position basedon setpoint commands, the monitored engine parameters, the modelcorrections and the engine state estimates. The air throttle is adjustedbased on the desired air throttle position, and the EGR valve isadjusted based on the desired EGR valve position.

Generating a desired air throttle position and a desired EGR valveposition can be performed using an inverse engine model that is aninverse of the engine observer model. The method can also includegenerating feedback actuator commands based on the setpoint commands andthe engine state estimates; and using the feedback actuator commands ingenerating the desired air throttle and EGR valve positions.

The air system can also include a charge air cooler and an engine intakemanifold coupled to the air throttle, such that intake air to the engineflows through the charge air cooler, the air throttle and the engineintake manifold. In this embodiment, the method can also includeestimating a pressure difference across the charge air cooler;estimating a temperature difference across the charge air cooler; andestimating an air throttle mass flow as part of the engine stateestimates based on the pressure and temperature differences across thecharge air cooler. Estimating a pressure difference across the chargeair cooler can include estimating the charge air cooler inlet pressure;estimating the intake manifold pressure; and estimating the charge aircooler outlet pressure by starting at the previous value of the chargeair cooler outlet pressure and searching a percentage of the rangebetween the charge air cooler inlet pressure and the intake manifoldpressure for the charge air cooler outlet pressure that makes the chargeair cooler mass flow and the air throttle mass flow the same or closestto the same.

The air system can also include an EGR cooler and an engine intakemanifold coupled to the EGR valve, recirculated exhaust gasses from theengine flowing through the EGR cooler, the EGR valve and the engineintake manifold. In this embodiment, the method can also includeestimating a pressure difference across the EGR cooler; estimating atemperature difference across the EGR cooler; and estimating an EGRvalve mass flow as part of the engine state estimates based on thepressure and temperature differences across the EGR cooler. Estimating apressure difference across the EGR cooler can include estimating the EGRcooler inlet pressure; estimating the intake manifold pressure; andestimating the EGR cooler outlet pressure by starting at the previousvalue of the EGR cooler outlet pressure and searching a percentage ofthe range between the EGR cooler inlet pressure and the intake manifoldpressure for the EGR cooler outlet pressure that makes the EGR coolermass flow and the EGR valve mass flow the same or closest to the same.

The generating model corrections step can include generating a fresh airmass flow correction term based on a pressure error at the engine intakemanifold, and/or generating an EGR mass flow correction term based onthe difference between the modelled and measured mass flows through theEGR cooler.

The method can also include calculating a desired intake manifolddiluent mass fraction based on a desired diluent-to-air ratio setpoint,an engine speed and a number of cylinders in the engine; calculating adesired EGR mass flow based on the desired intake manifold diluent massfraction; and calculating the desired EGR valve position based on thedesired EGR mass flow. The method can also include calculating a desiredintake manifold pressure based on the desired diluent-to-air ratiosetpoint and the number of cylinders in the engine; calculating adesired air throttle mass flow based on the desired intake manifoldpressure; and calculating the desired air throttle position based on thedesired air throttle mass flow.

The method can also include calculating a desired intake manifolddiluent mass fraction based on a desired diluent-to-air ratio setpoint,an engine speed and a number of cylinders in the engine; calculating anintake manifold EGR mass fraction and a closed loop EGR mass flow basedon the desired intake manifold diluent mass fraction; calculating amaximum air throttle mass flow and a maximum EGR valve mass flow;calculating a desired intake manifold pressure based on the desireddiluent-to-air ratio setpoint and the number of cylinders in the engine;calculating a maximum intake manifold mass flow based on the intakemanifold EGR mass fraction, the closed loop EGR mass flow, the maximumair throttle mass flow and the maximum EGR valve mass flow; calculatinga desired intake manifold mass flow in based on the maximum intakemanifold mass flow and the desired intake manifold pressure; calculatinga desired air throttle mass flow and a desired EGR valve mass flow basedon the intake manifold EGR mass fraction, the closed loop EGR mass flowand the desired intake manifold mass flow in; calculating the desiredEGR valve position based on the desired EGR valve mass flow; andcalculating the desired air throttle position based on the desired airthrottle mass flow.

A fresh air and exhaust gas control system is disclosed for an enginehaving an air system with an air throttle and an exhaust gasrecirculation (EGR) valve. The control system includes a plurality ofsensors, an engine observer, a comparator, an observer controller, andan inverse engine model. The plurality of sensors monitor engineparameters of the engine. The engine observer models the air system andgenerates engine state estimates. The comparator generates measuredengine states based on the engine parameters from the plurality ofsensors, and computes an observer error based on the differences betweenthe measured engine states and the engine state estimates. The observercontroller generates model corrections based on the observer error, andthe model corrections are input to the engine observer. The inverseengine model generates actuator commands based on setpoint commands, theengine parameters from the plurality of sensors, the model correctionsand the engine state estimates. The positions of the air throttle andthe EGR valve are adjusted based on the actuator commands.

The engine observer, the comparator, the observer controller, and theinverse engine model can run on an electronic control unit of theengine. The inverse engine model can include an inverse of the engineobserver.

The fresh air and exhaust gas control system can also include a feedbackcontroller that generates feedback actuator commands based on thesetpoint commands and the engine state estimates; and the positions ofthe aft throttle and the EGR valve would be adjusted based on theactuator commands and the feedback actuator commands.

The air system can include a charge air cooler and an engine intakemanifold coupled to the air throttle, such that intake air to the engineflows through the charge air cooler, the air throttle and the engineintake manifold. In this case, the engine state estimates generated bythe engine observer can include an air throttle mass flow estimate basedon pressure and temperature differences across the charge air cooler.

The air system can include an EGR cooler and an engine intake manifoldcoupled to the EGR valve, such that recirculated exhaust gasses from theengine flow through the EGR cooler, the EGR valve and the engine intakemanifold. In this case, the engine state estimates generated by theengine observer can include an EGR valve mass flow estimate based onpressure and temperature differences across the EGR cooler.

The observer controller can generate a fresh air mass flow correctionterm based on the difference between the modelled and measured intakemanifold pressures. The observer controller can generate an EGR massflow correction term based on the difference between the modelled andmeasured EGR cooler mass flows.

The above and other features will become apparent from the followingdescription and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description of the drawing refers to the accompanyingfigures in which:

FIG. 1 illustrates an exemplary observer based control system includinga physical process and a process model;

FIG. 2 illustrates some of the components, state variables, mass flows,and sensors used in an exemplary air system model;

FIG. 3 illustrates a high level view of an exemplary control method;

FIG. 4 illustrates an exemplary air throttle and EGR valve controlsystem;

FIG. 5 illustrates an exemplary method for calculating desired EGR valveand air throttle positions;

FIG. 6 illustrates another exemplary method for calculating desired EGRvalve and air throttle positions;

FIG. 7 illustrates an exemplary turbocharger control model;

FIG. 8 illustrates an exemplary method for calculating turbocharger vanepositions; and

FIG. 9 illustrates an exemplary two dimensional binary search forfinding turbine interstage pressure and desired vane position for agiven turbine mass flow.

DETAILED DESCRIPTION

Internal combustion engines use recirculated exhaust gas to reduceemissions of nitrogen oxides (NOx). Achieving the optimum combination ofair and burnt gasses within the engine cylinder prior to the start ofcombustion on each engine cycle may not be possible due to the dynamicsof the air system. A physics-based control strategy can be used thatprioritizes the control objectives of the air system and regulates theflow of fresh air and exhaust gas recirculation (EGR) under transientconditions. A state observer can be used to model the air system states,measured states can be compared to estimated states, and corrections canbe made to the observer model to minimize errors. The model informationand model corrections can be used in feed forward calculations todetermine the desired air throttle position and the desired EGR valveposition. Feed forward control achieved using this method can providefast and accurate control within the constraints of the system.

A state observer is a math model of a process that is being controlled.The model can be run in an electronic control unit (ECU) of an engine.FIG. 1 illustrates an exemplary observer based control system 100 thatincludes a physical process 102 and a process model 104. The processmodel 104 calculates state estimates, for example pressures,temperatures, speeds, and positions. Measured states from the physicalprocess 102 can be compared with the state estimates from the processmodel 104, and corrections can be made based on the differences.

The exemplary observer based control system 100 includes the physicalprocess 102 that is being modeled by the process model 104, as well asan observer controller 106, a feedback controller 108 and a feed forwardcontroller 110. The feed forward controller 110 is an inverse of theprocess model 104. The process model 104 is not a perfect model of thephysical process 102 which experiences disturbances 112 that are notaccounted for by the process model 104. Sensors monitoring the physicalprocess 102 provide a measured state 114, and the process model 104outputs a measured state estimate 116. The difference between themeasured state 114 and the measured state estimate 116 provides anobserver error 118 that is input to the observer controller 106. Theobserver controller 106 processes the observer error 118 and generates amodel correction 120 that is input to the process model 104 and to thefeed forward controller 110. Setpoint commands 122 indicating desiredvalues for process parameters are input to the control system 100. Thesetpoint commands 122 are input to the feed forward controller 110. Thedifference between the setpoint commands 122 and a controlled stateestimate 124 generated by the process model 104 produces a control error126 that is input to the feedback controller 108. The sum of the outputsof the feedback controller 108 and the feed forward controller 110produces actuator commands 128 that are input to the physical process102 and the process model 104.

There are several advantages to using a state observer within a controlsystem. The observer can provide estimates of states that are difficult,expensive, or impossible to measure directly. Since the process model104 provides state estimates, fewer sensors may be required. The processmodel 104 with corrections from the observer controller 106 can be usedin the feed forward calculation 110 for the actuator commands 128. Thisinverse process model 110 has desired states or setpoints 122 as theinputs, and the corresponding actuator commands 128 are the outputs.Feed forward control of this type provides fast response and can reducethe feedback control complexity. This method can make it easier toimplement system constraints because the constraints may be treated aslimits within the feed forward and feedback control eliminating the needfor separate controllers modifying the actuator commands. Using anobserver in the control system may also improve operation innon-standard conditions because the model can predict the effects ofchanges and adjust the controls as needed.

Since the observer model runs in the ECU, it needs to be efficient atcalculating state estimates. Finite element models and one-dimensionalwave dynamic models are typically too complex to run within the ECU atreal time. Models capable or running within the ECU or in off linesimulations with sufficient fidelity for use with a control system aretypically mean value lumped parameter models. A mean value modelcalculates the mass flow through the engine as being continuous withoutthe pulsating effects of a real engine. If needed, individual cylindermasses can be calculated for each cylinder event from the mean flow.

Flow into the engine cylinders can be predicted using the speed densitymethod which uses engine speed and engine displacement to determine anideal volumetric flow rate. A correction factor called the volumetricefficiency can then be applied to provide an estimate of the volumetricflow rate. Finally, the volumetric flow rate can be multiplied by theintake manifold density to determine the mass flow into the enginecylinders. This is sometimes called the speed density mass flow.

Engine manifolds and pipes can be modeled using the lumped parameter(zero dimensional) method. Using this method, the entire volume of themanifold is assumed to have uniform pressure, temperature, and mixturecomposition; all mass flows out of the manifold are assumed to have thesame pressure, temperature, and mixture composition as the contents ofthe manifold; and the pipes are assumed to have no wave dynamics ortransport delays.

FIG. 2 illustrates some of the components, state variables, mass flows,and sensors used in an exemplary air system model. Air enters the systemat a compressor inlet 200 into a low pressure compressor 202. Sensorscan be located at the compressor inlet 200 to monitor variousparameters, for example compressor inlet temperature, pressure andhumidity. The low pressure compressor 202 compresses the air and sendsit to a high pressure compressor 204 where it is further compressed. Thecompressed air from the high pressure compressor 204 is input to acharge air cooler (CAC) 206. Various parameters, for example, pressure,temperature and diluent mass fraction in the CAC 206 can be monitored.The output from the CAC 206 passes through an air throttle 208 and intoa mixer 210. Sensors can be positioned between the CAC 206 and the airthrottle 208 to monitor the CAC outlet temperature and pressure as wellas other parameters. A position sensor can be used to monitor theposition of the air throttle 208.

The mixer 210 mixes fresh air from the CAC 206 and recirculated exhaustgasses from an exhaust gas recirculation (EGR) cooler 212. The mixedgasses from the mixer 210 are fed to an engine intake manifold 216, andthe output of the engine intake manifold 216 is divided betweencylinders 242 of an engine 240. The illustration of FIG. 2 shows anengine 240 with six cylinders 242. Various parameters, for example,pressure, temperature and diluent mass fraction at the intake manifold216 can be monitored.

The exhaust gasses of the cylinders 242 of the engine 240 are fed to anengine exhaust manifold 218. The exhaust gasses from the exhaustmanifold 218 are either recirculated through the EGR cooler 212 orexpelled from the system. Various parameters, for example, pressure,temperature and diluent mass fraction at the exhaust manifold 218 can bemonitored.

The output from the EGR cooler 212 passes through an EGR valve 214 andinto the mixer 210. Various parameters, for example, pressure,temperature and diluent mass fraction in the EGR cooler 212 can bemonitored. Sensors can be positioned between the EGR cooler 212 and theEGR valve 214 to monitor the EGR cooler outlet temperature and pressureas well as other parameters. A position sensor can be used to monitorthe position of the EGR valve 214. An EGR flow Venturi can be locatedbetween the EGR valve 214 and the mixer 210.

The exhaust gasses expelled from the exhaust manifold 218 pass through ahigh pressure turbine 224 and a low pressure turbine 222. The highpressure turbine 224 can be a variable geometry turbocharger (VGT) witha vane position sensor to monitor the vane positions. Various otherparameters, for example, pressure, temperature, turbocharger speed canbe monitored. An exhaust back pressure at the output of the low pressureturbine 222 can also be monitored.

The system shown in FIG. 2 can be modeled by state equations that arebased on the conservation of mass, energy, and momentum. Somerelationships can be approximated using first order filters. Each stateequation describes a rate of change. The state value at any given timecan be determined by integration of the state equation. Within the ECU,each state can be given an initial condition and the state value at eachtime step of the controller can be updated using numeric integration.

The rate of change of mass in the charge air cooler (CAC) 206 can bemodeled by the difference between the rate of change of mass coming infrom the compressors 202, 204 and the rate of change of mass going outto the air throttle 208 as:

$\begin{matrix}{\frac{{dm}_{cac}}{dt} = {{\overset{.}{m}}_{cmp} - {\overset{.}{m}}_{at}}} & (1)\end{matrix}$

The rate of change of mass in the intake manifold 216 can be modeled bythe sum of the rate of change of mass coming in from the air throttle208 and the EGR valve 214 minus the rate of change of mass going intothe engine 240 as:

$\begin{matrix}{\frac{{dm}_{im}}{dt} = {{\overset{.}{m}}_{at} + {\overset{.}{m}}_{egr} - {\overset{.}{m}}_{{eng}\_{in}}}} & (2)\end{matrix}$

The rate of change of diluent mass in the intake manifold 216 can bemodeled as the sum of the rate of change in mass coming from the airthrottle 208 times the mass fraction of diluent in the charge air cooler206 (can assume same as the mass fraction of water in ambient air due tohumidity) plus the rate of change of mass coming in from the EGR valve214 times the mass fraction of diluent in the exhaust manifold 218,minus the rate of change of mass going into the engine 240 times themass fraction of diluent in the intake manifold 216 as:

$\begin{matrix}{\frac{{dm}_{d\_ im}}{dt} = {{{\overset{.}{m}}_{at} \cdot \chi_{d\_ cac}} + {{\overset{.}{m}}_{egr} \cdot \chi_{d\_ em}} - {{\overset{.}{m}}_{{eng}\_{in}} \cdot \chi_{d\_ im}}}} & (3)\end{matrix}$The term diluent is used to describe everything other than dry air (orfuel) that is included in a mixture.

The rate of change of mass in the exhaust manifold 218 can be modeled bythe rate of change of mass coming out of the engine 240 minus the rateof change of mass going into the EGR cooler 212 and the rate of changeof mass going into the turbines 222, 224 as:

$\begin{matrix}{\frac{{dm}_{em}}{dt} = {{\overset{.}{m}}_{eng\_ out} - {\overset{.}{m}}_{egr} - {\overset{.}{m}}_{trb}}} & (4)\end{matrix}$

The rate of change of diluent mass in the exhaust manifold 218 can bemodeled by the difference between the rate of change of mass coming infrom the engine 240 times the mass fraction of diluent coming in fromthe engine 240 and the sum of the rates of change of mass going into theEGR cooler 212 and the turbines 222, 224 times the mass fraction ofdiluent in the exhaust manifold 218 as:

$\begin{matrix}{\frac{d\; m_{d\;\_\;{em}}}{dt} = {{{\overset{.}{m}}_{{eng}\;\_\;{out}} \cdot \chi_{d\;\_\;{eng}\mspace{11mu}\_\;{out}}} - {( {{\overset{.}{m}}_{egr} + {\overset{.}{m}}_{trb}} ) \cdot \chi_{d\;\_\;{em}}}}} & (5)\end{matrix}$

The rate of change of temperature at the output of the CAC 206 can bemodeled by the difference between the steady state temperature at theoutput of the CAC 206 and the measured temperature at the output of theCAC 206 divided by a time constant for the CAC 206 as:

$\begin{matrix}{\frac{{dT}_{caco}}{dt} = \frac{( {T_{{caco}\;\_\;{ss}} - T_{caco}} )}{\tau_{{cac}\;}}} & (6)\end{matrix}$

The rate of change of temperature at the output of the EGR cooler 212can be modeled by the difference between the steady state temperature atthe output of the EGR cooler 212 and the measured temperature at theoutput of the EGR cooler 212 divided by a time constant for the EGRcooler 212 as:

$\begin{matrix}{\frac{{dT}_{egrco}}{dt} = \frac{( {T_{{egrco}\;\_\;{ss}} - T_{egrco}} )}{\tau_{egrc}}} & (7)\end{matrix}$

The steady state cooler outlet temperatures T_(caco_ss) and T_(egrco_ss)can be calculated using a heat exchanger effectiveness model. Theeffectiveness can be calibrated using a table with mass flow as theinput. An effectiveness of one means the cooler outlet temperature isequal to the temperature of the cooling fluid, and an effectiveness ofzero means there is no change in temperature between the cooler inletand outlet.

The rate of temperature change of the intake manifold 216 can becalculated using the mass flows in and out, the temperature in and out,and the change in mass within the intake manifold 216. The rate ofchange of temperature of the intake manifold 216 can be modeled by therate of change of mass at the air throttle 208 times C_(P) times thetemperature at the output of the CAC 206 plus the rate of change of massat the EGR valve 214 times C_(P) times the temperature at the output ofthe EGR cooler 212 minus the rate of change of mass going into theengine 240 times C_(P) times the temperature of the intake manifold 216minus the rate of change of mass in the intake manifold 216 times C_(V)times the temperature of the intake manifold 216, all divided by theproduct of the mass at the intake manifold 216 times C_(V) as:

$\begin{matrix}{\frac{{dT}_{i\; m}}{dt} = \frac{\begin{matrix}{{{\overset{.}{m}}_{at} \cdot C_{p} \cdot T_{caco}} + {{\overset{.}{m}}_{egr} \cdot C_{p} \cdot T_{egrco}} -} \\{{{\overset{.}{m}}_{{eng}\;\_\; i\; n} \cdot C_{p} \cdot T_{i\; m}} - {\frac{d\; m_{i\; m}}{dt} \cdot C_{v} \cdot T_{i\; m}}}\end{matrix}}{m_{i\; m} \cdot C_{v}}} & (8)\end{matrix}$where C_(P) is the specific heat at constant volume, and C_(V) is thespecific heat at constant pressure. The specific heat can be measuredexperimentally at constant volume or constant pressure. C_(P) is greaterthan C_(V) because as the mixture is heated at constant pressure itexpands doing work on the container or the fluid around it. C_(P) can beused to calculate energy flow into or out of the control volume. C_(V)can be used to calculate the change in energy within the control volumedue to changes in temperate and mass.

The rate of change of temperature at the exhaust manifold 218 can bemodeled by the difference between the temperature at the output of theengine 240 and the temperature of the exhaust manifold 218 divided by atime constant for the exhaust manifold 218 as:

$\begin{matrix}{\frac{{dT}_{em}}{dt} = \frac{( {T_{{eng}\;\_\;{out}} - T_{em}} )}{\tau_{em}}} & (9)\end{matrix}$

The rate of change in low pressure turbocharger speed can be modeled bythe sum of the torques on the low pressure turbocharger shaft and bladesdivided by the inertia of the low pressure turbocharger shaft and blades(Newton's second law for rotation). The sum of the torques can bemodeled by the difference between low pressure turbine power and lowpressure compressor power divided by the low pressure turbochargerspeed. The turbine power (or compressor power) can be calculated fromthe change in enthalpy of the gas as it goes through the turbine (orcompressor). The enthalpy change is equal to the mass flow rate timesthe specific heat at constant pressure (C_(p)) times the deltatemperature across the turbine (or compressor). When turbocharger speedis expressed in units of revolutions per minute (rpm), it must beconverted to radians per second (rad/s) using a factor of π/30. Theinverse of this factor is squared in equation (10) to convert N_(lpt) inthe denominator and also to express the solution dN_(lpt)/dt in units ofrevolutions per minute per second (rpm/s).

$\begin{matrix}{\frac{{dN}_{lpt}}{dt} = {\frac{1}{J_{lpt}} \cdot ( \frac{{\overset{.}{W}}_{{lp}\;\_\;{trb}} - {\overset{.}{W}}_{{lp}\;\_\;{cmp}}}{N_{lpt}} ) \cdot ( \frac{{30/\pi}\mspace{14mu}{rpm}}{{rad}\text{/}s} )^{2}}} & (10)\end{matrix}$

The rate of change in high pressure turbocharger speed can be modeled bythe sum of the torques on the high pressure turbocharger shaft andblades divided by the inertia of the high pressure turbocharger shaftand blades (Newton's second law for rotation). The sum of the torquescan be modeled by the difference between high pressure turbine power andhigh pressure compressor power divided by the high pressure turbochargerspeed. The turbine power (or compressor power) can be calculated fromthe change in enthalpy of the gas as it goes through the turbine (orcompressor). The enthalpy change is equal to the mass flow rate timesthe specific heat at constant pressure (C_(p)) times the deltatemperature across the turbine (or compressor). When turbocharger speedis expressed in units or revolutions per minute (rpm) it must beconverted to radians per second (rad/s) using a factor of π/30. Theinverse of this factor is squared in equation (11) to convert N_(hpt) inthe denominator and also to express the solution dN_(hpt)/dt in units ofrevolutions per minute per second (rpm/s).

$\begin{matrix}{\frac{{dN}_{hpt}}{dt} = {\frac{1}{J_{hpt}} \cdot ( \frac{{\overset{.}{W}}_{h\; p\;\_\;{trb}} - {\overset{.}{W}}_{h\; p\;\_\;{cmp}}}{N_{hpt}} ) \cdot ( \frac{{30/\pi}\mspace{14mu}{rpm}}{{rad}\text{/}s} )^{2}}} & (11)\end{matrix}$

The air throttle and EGR valve can be modeled using the compressible gasflow equation for an orifice as:

$\begin{matrix}{{{\overset{.}{m}}_{at} = {{C_{d\;\_\;{at}}( u_{at} )} \cdot A_{at} \cdot \frac{P_{caco}}{\sqrt{R \cdot T_{caco}}} \cdot {\Psi( \frac{P_{i\; m}}{P_{caco}} )}}},{and}} & (12) \\{{\overset{.}{m}}_{egr} = {{C_{d\;\_\;{egr}}( u_{egr} )} \cdot A_{egr} \cdot \frac{P_{egrco}}{\sqrt{R \cdot T_{egrco}}} \cdot {\Psi( \frac{P_{i\; m}}{P_{egrco}} )}}} & (13)\end{matrix}$where C_(d_at) and C_(d_egr) are the discharge coefficients for the airthrottle 208 and the EGR valve 214, respectively, which can becalibrated using a table with actuator position (u_(at) and u_(egr),respectively) as the input. The fresh air flow correction factorCF_(air) (calculated below in equation (26)) and the EGR correctionfactor CF_(egr) (calculated below in equation (32)) can be used tomultiply the result of the air throttle and EGR valve mass flowequations (12) and (13), respectively, as:

$\begin{matrix}{{{\overset{.}{m}}_{at} = {{CF}_{air} \cdot {C_{d\;\_\;{at}}( u_{at} )} \cdot A_{at} \cdot \frac{P_{caco}}{\sqrt{R \cdot T_{caco}}} \cdot {\Psi( \frac{P_{i\; m}}{P_{caco}} )}}},{and}} & ( {12A} ) \\{{\overset{.}{m}}_{egr} = {{CF}_{egr} \cdot C_{d\;\_\;{egr}} \cdot A_{egr} \cdot \frac{P_{egrco}}{\sqrt{R \cdot T_{egrco}}} \cdot {\Psi( \frac{P_{i\; m}}{P_{{egrco}\;}} )}}} & ( {13A} )\end{matrix}$The areas A can be calculated using the actuator bore diameters. Theterm R is the gas constant. The compressible gas flow factor Ψ is afunction of pressure ratio and can be calibrated using a table withpressure ratio as the input. The table values can be calculated off-lineusing the following equations:

$\begin{matrix}{{\Psi( \frac{P_{out}}{P_{i\; n}} )} = {{{( \frac{P_{out}}{P_{i\; n}} )^{\frac{1}{k}} \cdot \sqrt{\frac{2 \cdot k}{k + 1} \cdot ( {1 - ( \frac{P_{out}}{P_{i\; n}} )^{\frac{k - 1}{k}}} )}}\mspace{14mu}{{for}( \frac{P_{out}}{P_{i\; n}} )}} \geq ( \frac{2}{k + 1} )^{\frac{k}{k - 1}}}} & (14) \\{{\Psi( \frac{P_{out}}{P_{i\; n}} )} = {{\sqrt{k \cdot ( \frac{2}{k + 1} )^{\frac{k + 1}{k - 1}}}\mspace{14mu}{for}\mspace{14mu}( \frac{P_{out}}{P_{i\; n}} )} < ( \frac{2}{k + 1} )^{\frac{k}{k - 1}}}} & (15)\end{matrix}$where the term k represents the ratio of specific heats C_(p)/C_(v).

The mass flow calculation for the charge aft cooler 206 and the EGRcooler 212 can be based on the Darcy-Weisbach equation:

$\begin{matrix}{h_{f} = {f \cdot \frac{L}{d} \cdot \frac{V^{2}}{2 \cdot g}}} & (16)\end{matrix}$This relationship was developed for a pipe, but the structure of thecoolers 206, 212 are similar. The term h_(f) is the pipe head loss, f isthe Darcy friction factor, L is the pipe length, d is the diameter, V isvelocity, and g is the gravitational constant.

Pressure drop is related to the head loss by:ΔP=ρ·g·h _(f)  (17)Substituting (16) into (17) results in:

$\begin{matrix}{{\Delta\; P} = {f \cdot \frac{L}{d} \cdot \frac{\rho \cdot V^{2}}{2}}} & (18)\end{matrix}$Velocity and mass flow are related by:

$\begin{matrix}{V = \frac{\overset{.}{m}}{\rho \cdot A}} & (19)\end{matrix}$Substituting (19) into (18) results in:

$\begin{matrix}{{\Delta\; P} = {f \cdot \frac{L}{d} \cdot \frac{1}{2 \cdot A^{2}} \cdot \frac{{\overset{.}{m}}^{2}}{\rho}}} & (20)\end{matrix}$

Equation (20) shows a relationship between mass flow squared overdensity and the pressure drop across the cooler. The remaining terms areconstant. An empirical model of this form can be fit to experimentaldata. The experimental data may also show a relationship to temperaturechange across the cooler. This finding is reasonable because the densityis changing as the gasses flow through the cooler and not constant asindicated by equation (20). These relationships can be calibrated usinga regression model or tables of the following form, where delta pressureand delta temperature are the inputs:

$\begin{matrix}{\frac{{\overset{.}{m}}_{cac}^{2}}{\rho_{{cac}\;\_\;{avg}}} = {{CACMassFlowSquaredOverDensityTable}( {{\Delta\; P_{cac}},{\Delta\; T_{cac}}} )}} & (21) \\{\frac{{\overset{.}{m}}_{egrc}^{2}}{\rho_{{egrc}\;\_\;{avg}}} = {{EGRCMassFlowSquaredOverDensityTable}( {{\Delta\; P_{egrc}},{\Delta\; T_{egrc}}} )}} & (22)\end{matrix}$

The inlet and outlet temperatures of each cooler 206, 212 can be modeledand/or the temperatures can be measured. Half of the cooler can beassumed to be at the inlet temperature and half of the cooler can beassumed to be at the outlet temperature. The average cooler density canbe calculated using the average temperature and average pressure.Assuming a linear drop in pressure and temperate across the cooler, theaverage temperature and average pressure can be calculated by averaginginlet and outlet values. The mass flow can be calculated by multiplyingthe average density by the table output and then taking the square root.The fresh air flow correction factor CF_(air) (calculated below inequation (26)) and the EGR correction factor CF_(egr) (calculated belowin equation (32)) can be included in these mass flow calculations forthe coolers as:

$\begin{matrix}{{\overset{.}{m}}_{cac} = {{CF}_{air} \cdot \sqrt{\rho_{{cac}\_{avg}} \cdot {{CACFlowSqOverDensityTbl}( {{\Delta\; P_{cac}},{\Delta\; T_{cac}}} )}}}} & ( {21A} ) \\{{\overset{.}{m}}_{egrc} = {{CF}_{egr} \cdot \sqrt{\rho_{{egrc}\_{avg}} \cdot {{EGRCFlowSqOverDensityTbl}( {{\Delta\; P_{egrc}},{\Delta\; T_{egrc}}} )}}}} & ( {22A} )\end{matrix}$

The mass flow through the charge air cooler 206 and the air throttle 208can be assumed to be the same. The outlet pressure of the charge aircooler 206 can be estimated using a binary search algorithm thatsearches for a charge air cooler outlet pressure that results in themass flow of the charge air cooler 206 and the mass flow of the airthrottle 208 being the same. The outlet pressure of the charge aircooler 206 should be between the inlet pressure of the charge air cooler206 and the pressure of the intake manifold 216. This entire range doesnot have to be searched at every ECU time step. To reduce processorload, the previous solution can be used as a starting point and only asmall range searched around that value. Even though the solution may notbe within the search range under extreme transient conditions, thesolution should converge within a few ECU time steps.

The calculation can begin by evaluating mass flow through the charge aircooler 206 and the air throttle 208 with the previous outlet pressure ofthe charge air cooler 206. If the mass flow of the air throttle 208 islower than the mass flow of the charge air cooler 206, then the outletpressure of the charge air cooler 206 can be adjusted up by a percentageof the maximum search range (charge air cooler inlet pressure-intakemanifold pressure). Otherwise, the outlet pressure of the charge aircooler 206 can be adjusted down by a percentage of the maximum searchrange. This process can be repeated with percentage adjustments of, forexample, 4, 2, 1, 0.5 and 0.25 percent of the maximum search range(charge air cooler inlet pressure-intake manifold pressure).

The same type of iterative solution technique can be used for the massflow through the EGR cooler 212 and the EGR valve 214.

The compressors 202, 204 and turbines 222, 224 can be modeled using massflow tables and efficiency tables (turbocharger maps). The pressureratio across the compressors (or turbines) and the turbocharger speedcan be used to determine the mass flow and efficiency. Thesecalculations can include corrections for temperature and pressure whennot operating at the standard conditions for the turbocharger maps. Theefficiency can be used to calculate the outlet temperature and theturbocharger shaft power. The turbocharger speed can be calculated usingthe compressor power, turbine power, and inertia as shown by equations(10) and (11).

A variable geometry turbocharger (VGT) 224 has adjustable vanes on theturbine that change the turbine performance characteristics. Moving thevanes in the closing direction causes the pressure of the exhaustmanifold 218 to increase. Up to some point this also causes the turbinepower to increase, which causes the turbocharger to speed up and providemore compressor flow. If the turbine vanes are closed too much theturbine flow can be “choked” and the power may drop.

To model the turbine operation at various vane positions, multipleturbine tables can be used that characterize the turbine performance atdifferent vane positions. For operation at vane positions between thetables, interpolation can be used to determine the mass flow andefficiency.

Two stage turbochargers have two compressors in series and two turbinesin series, as shown in FIG. 2. The interstage volume between thecompressors 202, 204 and between the turbines 222, 224 is small, makingit difficult to dynamically model the pressure. A small change in flowcauses a large change in pressure. At the time steps used in the ECU,the modeled interstage pressure can become numerically unstable. A flowchange over one time step can cause a change in pressure that in turncauses another change in flow and pressure causing the modelledpressures and flows to oscillate. This can be overcome by using a verysmall time step but that is not typically practical for calculationswithin an ECU.

To eliminate the numeric instability, the flow through each compressor202, 204 can be assumed to be the same. Likewise, the flow through eachturbine 222, 224 can be assumed to be the same. The interstage pressurecan then be solved iteratively using a binary search algorithm thatadjusts the interstage pressure until the flow through each compressor(or turbine) is the same. This is similar to the search algorithmdescribed above for the charge air cooler 206 and the air throttle 208.

The observer controller can make corrections to the air system model sothat the estimated states match the measured states. The modeled statesmay have error due to modeling errors, engine variation, changing enginecharacteristics over time, air leaks and other sources. Three examplesof model corrections are: fresh air mass flow correction term, EGR massflow correction term, and turbine mass flow correction term. Thesecorrection terms can be calculated within the observer controller basedon: intake manifold pressure error, EGR mass flow error, and exhaustmanifold pressure error, respectively. The model corrections can bemultiplicative correction factors, additive or offset correction factorsor other types of correction terms. Exemplary correction termcalculations are shown below. The correction terms can be applied to theobserver model as well as to the models used in actuator controlcalculations.

A fresh air flow correction factor CF_(air) can be applied as acorrection to the compressor mass flow estimates, charge air cooler massflow estimates, and air throttle mass flow estimates. The pressure errorat the intake manifold 216 is the difference between the modelled andmeasured pressures:P _(im_obs_error) =P _(im_sensor) −P _(im)  (23)A proportional air flow correction factor CF_(air_prop) can becalculated as a constant times the pressure error at the intake manifold216:CF_(air_prop) =K _(p_im_obs) ·P _(im_obs_error)  (24)An incremental air flow intake correction factor CF_(air_int) can alsobe calculated based on the pressure error at the intake manifold 216 as:CF_(air_int) =K _(i_im_obs) ·P_(im_obs_error)+CF_(air_int_previous)  (25)A fresh air flow correction factor CF_(air) can be estimated by the sumof these air flow correction factors:CF_(air)=CF_(air_prop)+CF_(air_int)  (26)

A turbine mass flow correction factor CF_(trb) can be applied as acorrection to the turbine mass flow estimates. The pressure error at theexhaust manifold 218 is the difference between the modelled and measuredpressures:P _(em_obs_error) =P _(em_sensor) −P _(em)  (27)A proportional turbine correction factor CF_(trb_prop) can be calculatedas a constant times the pressure error at the exhaust manifold 218:CF_(trb_prop) =−K _(p_em_obs) ·P _(em_obs_error)  (28)An incremental turbine intake correction factor CF_(trb_int) can also becalculated based on the pressure error at the exhaust manifold 218 as:CF_(trb_int) =−K _(i_em_obs) ·P_(em_obs_error)+CF_(trb_int_previous)  (29)

A turbine intake correction factor CF_(trb) can be estimated by the sumof these turbine correction factors:CF_(trb)=CF_(trb_prop)+CF_(trb_int)  (30)

An EGR mass flow correction factor CF_(egr) can be applied as acorrection to the EGR cooler mass flow estimates, and the EGR valve massflow estimates. A mass flow sensor can be located between the EGR valve214 and the mixer 210, and the mass flow error at the EGR cooler 212 canbe estimated as the difference between the modelled and measured massflows:{dot over (m)} _(egr_obs_error) ={dot over (m)} _(egr_sensor) −{dot over(m)} _(egr)  (31)An EGR Flow Venturi can be located between the EGR valve 214 and themixer 210 to measure the EGR mass flow. The pressure drop from the inletto the throat of the Venturi (delta pressure) is related to the massflow rate. Some engines can use an EGR flow orifice, and the measuredpressure ratio across the orifice can be related to the flow rate (e.g.,using equation 13). An incremental EGR correction factor CF_(egr) can becalculated based on this mass flow error as:CF_(egr) =K _(i_egr_obs) ·{dot over (m)}_(egr_obs_error)+CF_(egr_previous)  (32)

Proportional plus integral (PI) controllers can be used for the intakemanifold pressure and exhaust manifold pressure control. An integralcontroller can be used for the EGR mass flow error control. The EGR massflow correction factor can be applied directly to the EGR mass flowestimate to make integral control sufficient.

Fuel reacts with oxygen during combustion. When fuel reacts with aquantity of air that has just enough oxygen to combine with all the fuelmolecules to form combustion products of carbon dioxide and water, thatratio of fuel to air is called the stoichiometric ratio. Diesel enginestypically run leaner than the stoichiometric ratio meaning they haveexcess air in the cylinder. This excess air passes through the exhaustvalves into the exhaust with the burnt gasses.

Engines do not typically run on only dry air. The air that enters theengine may have some water vapor. In addition, modern engines commonlyuse exhaust gas recirculation (EGR) to reduce combustion temperaturesfor controlling nitrogen oxides (NOx) emissions. Recirculated exhaustgas may contain combustion products as well as air.

The term “diluent” is used to describe everything other than dry air (orfuel) that is included in a mixture. The mass fraction of oxygen in dryair is 0.232. If the oxygen mass fraction in a mixture is known, the airmass fraction can be determined by dividing by 0.232. If there is nofuel, the diluent mass fraction can be calculated by taking one minusthe air mass fraction.

Within an air system state observer, the diluent mass fraction can becalculated for the charge air cooler, intake manifold, and exhaustmanifold. The diluent mass fraction can be chosen as a variable forconvenience because it increases with increasing EGR for the intake andexhaust manifolds. For the charge air cooler, the diluent mass fractionincreases with increasing ambient relative humidity. The same modelingresults can be obtained using dry air mass fraction or oxygen massfraction as state variables.

Another way to describe the diluent concentration is as a ratio ofdiluent to air (D/A). D/A can be used as a variable for the in-cylinderconcentration to be consistent with fuel-to-air ratio (F/A), which iscommonly used for engine control.

Two important parameters in air system control are the in-cylinderdiluent-to-air ratio (D/A) and fuel-to-air ratio (F/A). D/A iscontrolled to regulate NOx. F/A is controlled to prevent rich operationcausing smoke, and to prevent lean operation causing high cylinderpressure. These ratios can be calculated from the desired fuel percylinder, the air mass in-cylinder when the intake valve closes, and thediluent mass in-cylinder when the intake valve closes. The in-cylinderair mass has two components: air that is present in the residual massfrom the previous engine cycle, and air that entered through the intakevalve on the current engine cycle:m _(a_cyl) =m _(a_cyl_in) +m _(a_res)  (33)Likewise the in-cylinder diluent mass has two components: diluent thatis present in the residual mass from the previous engine cycle, anddiluent that entered through the intake valve on the current enginecycle.m _(d_cyl) =m _(d_cyl_in) +m _(d_res)  (34)The in-cylinder fuel-to-air ratio (F/A) is then:

$\begin{matrix}{{F/A} = \frac{m_{f\_{des}}}{m_{a\_{cyl}}}} & (35)\end{matrix}$and the in-cylinder diluent-to-air ratio (D/A) is:

$\begin{matrix}{{D/A} = \frac{m_{d\_{cyl}}}{m_{a\_{cyl}}}} & (36)\end{matrix}$

A model can be used to predict the residual air mass and residualdiluent mass. The speed density mass flow model and the intake manifolddiluent mass fraction can be used to estimate the air mass and diluentmass that enters through the valves.

FIG. 3 illustrates a high level view of an exemplary control system 300.In this exemplary system, information is used to convert the desiredin-cylinder states to manifold states, the desired flow rates to achievethose states are then calculated, and then the flow models and modelcorrections are used to calculate the actuator positions. The controlsystem 300 includes an engine 302, an engine observer model 304, anobserver controller 306, a proportional feedback controller 308, a feedforward or inverse engine model 310 and a flow-to-position module 312.

The physical engine 302 experiences disturbances 340 that are notmodeled by the observer model 304. Sensors monitoring the engine 302provide a measured state 314. The engine observer model 304 processesvarious inputs (including, for example, speeds, masses, pressures,temperatures, model corrections, etc.) and generates a measured stateestimate 316 to be compared with the measured state 314, as well ascontrolled state estimates 324 that are provided to the inverse enginemodel 310 and the flow-to-position module 312. The difference betweenthe measured state 314 and the measured state estimate 316 provides anobserver error 318 that is input to the observer controller 306.

The observer controller 306 generates model corrections 320 that areprovided to the engine observer model 304, the inverse engine model 310and the flow-to-position module 312. The model corrections generated bythe observer controller 306 can include an integral term that drives thesteady state error to zero. This can help keep operation of the engineobserver model 304 close to that of the real engine 302. Another optionwould be to only use proportional control in the observer controller 306and have a proportional plus integral (PI) controller for the mainfeedback control 308.

The inverse engine model or feed forward controller 310 takes thecontrolled state estimates 324 generated by the engine observer model304, the model corrections 320 generated by the observer controller 306,desired state inputs 322 and various system parameters 326 andcalculates desired engine state commands 330 and feed forward mass flowterms 332 to achieve the desired D/A and F/A values included in thedesired state inputs 322.

The difference between the controlled state estimates 324 generated bythe engine observer model 304 and the desired engine state commands 330generated by the inverse engine model 310 is input to the feedbackcontroller 308 which calculates proportional flow commands 334. Theproportional feedback controller 308 can be used for transient control.To change a manifold pressure more or less mass has to be stored in themanifold. This change in mass storage can be achieved using theproportional controller 308. Using the proportional controller 308 canprovide smoother operation than using a derivative term in the feedforward calculation. The proportional feedback controller 308 also canremove error that may not be covered by the model correction factorsbecause of the response time or limits that may be applied to thecorrection factors.

The sum of the feed forward mass flows 332 generated by the inverseengine model 310 and the proportional flow commands 334 generated by theproportional feedback controller 308 provides flow commands 336 that areinput to the flow-to-position module 312. The flow-to-position module312 also receives inputs of the model corrections 320 generated by theobserver controller 306 and the controlled state estimates 324 generatedby the engine observer model 304. The flow-to-position module 312 takesthese inputs and an exhaust back pressure and calculates actuatorposition commands 328 that are input to the engine 302 and the engineobserver model 304.

FIG. 4 illustrates an exemplary air throttle and EGR valve controlsystem 400 which includes a charge air cooler (CAC) 406, an air throttle408, an intake manifold 416, an EGR valve 414, an EGR cooler 412 and acylinder 442. Only one cylinder 442 is shown for clarity but the enginewould include multiple cylinders 442 and the output from the intakemanifold 416 would be distributed across the multiple cylinders 442.

The air throttle 408 and EGR valve 414 provide fast response forcontrolling the diluent mass fraction and pressure of the intakemanifold 416. The turbocharge vanes (see VGT 224 of FIG. 2) can also beadjusted to control the EGR flow and fresh air flow but this control isslower. Control of the turbocharger is typically for providingsufficient exhaust manifold pressure and charge air cooler pressure sothat the air throttle 408 and EGR valve 414 can be adjusted to achievethe desired setpoints.

The system can be designed to give the diluent-to-air-ratio (D/A)control higher priority than the fuel-to-air-ratio (F/A) control. D/A iscontrolled by regulating the diluent mass fraction of the intakemanifold 416. F/A is controlled by regulating the pressure of the intakemanifold 416. In general, D/A control is faster than F/A control.

The desired diluent concentration in the intake manifold 416 can becalculated using the current in cylinder air mass instead of a desiredvalue calculated from the F/A setpoint because under transientconditions it may take a long time to achieve the F/A setpoint. FIG. 5shows a block diagram for the control.

The desired diluent mass fraction of the intake manifold 416 can becalculated at block 510 using the following exemplary method. The EGRcontrol calculation can calculate the mass of diluent that is desiredin-cylinder using the estimated current air mass in-cylinder and thedesired D/A setpoint:m _(d_cyl_des) =m _(a_cyl) *D/A _(des)  (37)The mass of diluent to enter the cylinder can then be calculated bysubtracting the residual diluent mass from the desired in-cylinderdiluent mass:m _(d_cyl_in_des) =m _(d_cyl_des) −m _(d_res)  (38)The desired diluent mass flow into the engine can then be calculatedusing engine speed and the number of cylinders:

$\begin{matrix}{{\overset{.}{m}}_{{d\_{eng}}{\_{in}}{\_{des}}} = {m_{{d\_{cyl}}{\_{in}}{\_{des}}} \cdot n_{cyl} \cdot {N_{e}({rpm})} \cdot ( \frac{\min}{60\mspace{14mu}\sec} ) \cdot ( \frac{cycle}{2\mspace{14mu}{rev}} )}} & (39)\end{matrix}$The desired intake manifold diluent mass fraction can then be calculatedby dividing the desired engine-in diluent mass flow by the speed densitymass flow:

$\begin{matrix}{\chi_{{d\_{im}}{\_{des}}} = \frac{{\overset{.}{m}}_{{d\_{eng}}{\_{in}}{\_{des}}}}{{\overset{.}{m}}_{{eng}\_{in}}}} & (40)\end{matrix}$

The desired mass flow through the EGR valve 414 can be calculated atblock 512 using the following exemplary method. The following equationcan be used to relate the desired diluent mass fraction of the intakemanifold 416 to the feed forward intake manifold EGR mass fraction:χ_(d_im_des)=χ_(d_em)·χ_(egr_im_ff)+χ_(d_cac)·(1−χ_(egr_im_ff))  (41)The mass fraction of diluent in the charge air cooler 406 can be used inthis calculation to account for water vapor that may enter with thefresh air. Water vapor is considered a diluent. This equation can bere-arranged to solve for the feed forward intake manifold EGR massfraction as:

$\begin{matrix}{\chi_{{{egr}\_{im}}{\_{ff}}} = \frac{\chi_{{d\_{im}}{\_{des}}} - \chi_{d\_{cac}}}{\chi_{d{\_ e}m} - \chi_{d\_{cac}}}} & (42)\end{matrix}$The feed forward EGR mass flow can be calculated by multiplying the feedforward intake manifold EGR mass fraction by the engine in mass flowestimate:{dot over (m)} _(egr_ff)=χ_(erg_im_ff) ·{dot over (m)} _(eng_in)  (43)The intake manifold diluent mass fraction error is the differencebetween the desired intake manifold diluent mass fraction and the intakemanifold diluent mass fraction estimate:χ_(d_im_error)=χ_(d_im_des)−χ_(d_im)  (44)The error can be multiplied by the proportional gain to determine theclosed loop term by:{dot over (m)} _(egr_cl) =K _(p_egr)·χ_(d_im_error)  (45)K_(p_egr) can be calibrated using a table with exhaust manifold diluentmass fraction as the input. This allows higher gains to be used when theexhaust manifold diluent mass fraction is low. The desired mass flowthrough the EGR valve 414 is the sum of the feed forward term and theclosed loop term:{dot over (m)} _(egr_des) ={dot over (m)} _(egr_ff) +{dot over (m)}_(egr_cl)  (46)The calculation of the desired position of the EGR valve 414 at block514 will be described below along with the calculation of the desiredposition of the air throttle 408 at block 524.

The air throttle feed forward command can be calculated using the enginein mass flow estimate and the EGR valve mass flow estimate as:{dot over (m)} _(at_ff) ={dot over (m)} _(eng_in) −{dot over (m)}_(egr)  (47)This air throttle feed forward term can be used to achieve a flow thatwill keep the pressure of the intake manifold 416 constant. The closedloop term that will be calculated next is used to change the pressure ofthe intake manifold 416.

The desired air mass in-cylinder can be calculated using the desiredfuel command and the desired F/A lean setpoint as:

$\begin{matrix}{m_{{a\_{cyl}}{\_ lean}{\_ des}} = \frac{m_{f\_{des}}}{F/A_{{lean}\_{des}}}} & (48)\end{matrix}$A lean setpoint can be used here because the air throttle command isonly used to close the air throttle 408 when the engine operationbecomes too lean. The turbocharger can use a F/A rich setpoint toincrease the engine boost when the engine operation becomes too rich.Having this control deadband between the rich and lean setpoints canallow the air throttle and turbocharger vanes to remain more open moreof the time than they would be if controlling to one F/A setpoint.

The desired pressure of the intake manifold 416 can be calculated atblock 520 using the following exemplary method. The desired diluent massin-cylinder can be calculated using the desired in-cylinder lean airmass and the desired D/A setpoint as:m _(d_cyl_lean_des) =m _(a_cyl_lean_des) ·D/A _(des)  (49)The mass of air to enter the cylinder can then be calculated bysubtracting the residual air mass from the desired in-cylinder air mass:m _(a_cyl_in_lean_des) =m _(a_cyl_lean_des) −m _(a_res)  (50)The mass of diluent to enter the cylinder can be calculated bysubtracting the residual diluent mass from the desired in-cylinderdiluent mass:m _(d_cyl_in_lean_des) =m _(d_cyl_lean_des) −m _(d_res)  (51)The total mass to enter the cylinder can be calculated as the sum of theair mass to enter the cylinder and the diluent mass to enter thecylinder:m _(cyl_in_lean_des) =m _(a_cyl_in_lean_des) +m_(d_cyl_in_lean_des)  (52)The pressure needed to achieve this mass can be found using the idealgas law and the volumetric efficiency by:

$\begin{matrix}{P_{{{im}\_{lean}}{\_{des}}} = \frac{n_{cyl} \cdot m_{{{cyl}\_{in}}{\_{lean}}{\_{des}}} \cdot R \cdot T_{im}}{V_{d} \cdot {VE}}} & (53)\end{matrix}$This equation is related to the speed-density calculation but does notaccount for speed because mass per cylinder is used instead of a massflow. The desired intake manifold lean pressure may be limited to acertain range for component protection and engine performance. Lowpressures are sometimes avoided to prevent high oil consumption and poorcombustion quality.

The desired mass flow through the air throttle 408 can be calculated atblock 522 using the following exemplary method. The intake manifoldpressure lean error is the difference between the desired intakemanifold lean pressure and the intake manifold pressure estimate:P _(im_lean_error) =P _(im_lean_des) −P _(im)  (54)The error can be multiplied by the proportional gain to determine theclosed loop term:{dot over (m)} _(at_cl) =K _(p_im) ·P _(im_lean_error)  (55)The desired air throttle mass flow is the sum of the feed forward termfrom equation (47) and the closed loop term from equation (55):{dot over (m)} _(at_des)={dot over (m)}_(at_ff) +{dot over (m)}_(at_cl)  (56)

The control described above and shown in FIG. 5 uses the EGR valve 414to control the intake manifold diluent mass fraction and the airthrottle 408 to control the intake manifold pressure. Under transientconditions when there is insufficient engine delta pressure to providethe required EGR mass flow the resulting intake manifold diluent massfraction may be too low. An alternative control strategy can be used toresolve this issue by using the air throttle 408 to control the freshair flow entering the intake manifold 416 when needed to achieve thedesired intake manifold diluent mass fraction. This is different in thatit calculates the maximum mass flow for the air throttle 408 and EGRvalve 414, then determines the maximum combined flow that is possiblewhile achieving the desired intake manifold diluent mass fraction. Withthis method, the intake manifold pressure is controlled by calculating acombined mass flow term that is within the maximum limit. The individualmass flows for the air throttle 408 and the EGR valve 414 are thencalculated as shown in FIG. 6. Many of the calculations are the same asdescribed above, and the differences are described below.

The desired intake manifold diluent mass fraction can be calculated atblock 602 using the method disclosed with equation (40). The feedforward intake manifold EGR mass fraction can be calculated at block 604using the method disclosed with equation (42). The closed loop EGR massflow can be calculated at block 604 using the method disclosed withequation (45). The desired intake manifold pressure can be calculated atblock 608 using the method disclosed with equation (53).

The maximum air throttle and EGR valve mass flows can be calculated atblock 606 using the maximum air throttle and EGR valve positions inequation (12) and (13), and equations (21) and (22). An iterativeprocess can be used to find the mass flow since the corresponding cooleroutlet pressure must be found that provides the same flow through theactuator and cooler. A similar iterative process can be used to the onedescribed above for the charge air and EGR cooler mass flows for theobserver, except that the maximum actuator position could be usedinstead of the current actuator position. The maximum actuator positiondoes not have to correspond to a wide open valve position. The maximumactuator position may be calibrated to a value less than wide open undersome operating conditions.

The maximum mass flow rate at the intake manifold 416 can be based onthe EGR valve mass flow limit calculated using the maximum EGR valveposition in equation (13), equation (22), the closed loop EGR mass flowterm from equation (45), and the feed forward intake manifold EGR massfraction from equation (42) as:

$\begin{matrix}{{\overset{.}{m}}_{{{im}\_{in}}{\_\max}{\_{egr}}{\_{ltd}}} = \frac{{\overset{.}{m}}_{{egr}\_\max} - {\overset{.}{m}}_{{egr}\_{cl}}}{\chi_{{{egr}\_{im}}{\_{ff}}}}} & (57)\end{matrix}$The closed loop EGR mass flow is subtracted from the maximum EGR valvemass flow to help ensure there is room to add it later as shown inequation (63).

The maximum intake manifold mass flow based on the air throttle massflow limit can be calculated using the maximum air throttle position inequation (12), equation (21), the closed loop EGR mass flow term fromequation (45), and the feed forward intake manifold EGR mass fractionfrom equation (42) as:

$\begin{matrix}{{\overset{.}{m}}_{{{im}\_{in}}{\_\max}{\_{at}}{\_{ltd}}} = \frac{{\overset{.}{m}}_{{at}\_\max} + {\overset{.}{m}}_{{egr}\_{cl}}}{( {1 - \chi_{{{egr}\_{im}}{\_{ff}}}} )}} & (58)\end{matrix}$The closed loop EGR mass flow is added to the maximum air throttle massflow because later it will be subtracted as shown in equation (64).

The maximum intake manifold mass flow can be calculated at block 610 asthe minimum of the two values calculated by equations (57) and (58):{dot over (m)} _(im_in_max)=min({dot over (m)} _(im_in_max_egr_ltd),{dot over (m)} _(im_in_max_at_ltd))  (59)

The feed forward intake manifold mass flow term can be equated to theengine in mass flow estimate:{dot over (m)} _(im_in_ff) ={dot over (m)} _(eng_in)  (60)The closed loop intake manifold mass flow term can be estimated as aproportional gain times the intake manifold pressure lean error that wascalculated in equation (54):{dot over (m)} _(im_in_cl) =K _(p_im) ·P _(im_lean_error)  (61)The desired intake manifold mass flow in calculated at block 612 can beestimated as the sum of the feed forward term and the closed loop term:{dot over (m)} _(im_in_des) ={dot over (m)} _(im_in_ff) +{dot over (m)}_(im_in_cl)  (62)

The desired EGR valve mass flow and air throttle mass flow can becalculated at block 614 from the desired intake manifold mass flow as:{dot over (m)} _(egr_des) ={dot over (m)} _(im_in_des)·χ_(egr_im_ff)+{dot over (m)} _(egr_cl)  (63){dot over (m)} _(at_des) ={dot over (m)}_(im_in_des)·(1−χ_(egr_im_ff))−m _(egr_cl)  (54)The feed forward intake manifold EGR mass fraction and the closed loopEGR mass flow terms do not affect the total mass flow entering themanifold, only the split between the EGR valve and the air throttle.

The desired position for the air throttle 408 and the desired positionfor the EGR valve 414 can be calculated from the desired flow rate foreach actuator. The cooler inlet pressure and intake manifold pressurecan be used in each case.

For the air throttle 408, the pressure drop across the CAC 406 can beevaluated using equation (21). This can be done by searching theCACMassFlowSquaredOverDensityTable described above to find the deltapressure that gives the desired mass flow. Alternatively, a separatetable can be used with delta pressure as the output and mass flowsquared over density as one of the inputs. The outlet pressure of theCAC 406 can then be calculated by subtracting this pressure drop fromthe CAC inlet pressure.

The pressure ratio across the air throttle 408 can be calculated bydividing the pressure of the intake manifold 416 by the outlet pressureof the CAC 406. Equation (12) can then be used to solve for thedischarge coefficient C_(d_at). The discharge coefficient C_(d_at) isrelated to actuator position u_(at). A table defining this relationshipwas described for equation (12) of the observer model. The table forequation (12) could have two columns (u_(at) and C_(d_at)), where u_(at)is the input and C_(d_at) is the output. For the feed forwardcalculation a similar table can be used, but in this case C_(d_at) isthe input and u_(at) is the output.

For the EGR valve 414, the pressure drop across the EGR cooler 412 canbe evaluated using equation (22). This can be done by searching theEGRCMassFlowSquaredOverDensityTable described above to find the deltapressure that gives the desired mass flow. Alternatively, a separatetable can be used with delta pressure as the output and mass flowsquared over density as one of the inputs. The outlet pressure of theEGR cooler 412 can then be calculated by subtracting this pressure dropfrom the EGR cooler inlet pressure.

The pressure ratio across the EGR valve 414 can be calculated bydividing the pressure of the intake manifold 416 by the outlet pressureof the EGR cooler 412. Equation (13) can then be used to solve for thedischarge coefficient C_(d_egr). The discharge coefficient C_(d_egr) isrelated to actuator position u_(egr). A table defining this relationshipwas described for equation (13) of the observer model. The table forequation (13) could have two columns (u_(egr) and C_(d_egr)), whereu_(egr) is the input and C_(d_egr) is the output. For the feed forwardcalculation a similar table can be used, but in this case C_(d_egr) isthe input and u_(egr) is the output.

Within these calculations the desired mass flows can be modified bydividing by the corresponding mass flow correction factors that werecalculated for the air system observer. This allows the correct positionto be calculated for the desired mass flow.

The turbocharger control can provide sufficient exhaust pressure toachieve a desired EGR mass flow and provide sufficient intake manifoldpressure to achieve a desired fuel-to-air ratio. The vanes on theturbocharger can be adjusted to regulate the mass flow through theturbine. Moving the vanes in the closing direction tends to reduce themass flow, increase the exhaust manifold pressure, and increase theturbine power. A flow based strategy can be used with the turbochargercontrol. The desired turbine mass flow can be calculated and then theturbocharger maps can be used to calculate the vane position needed toachieve the desired mass flow.

FIG. 7 illustrates an exemplary turbocharger control schematic 700 whichincludes a cylinder 742, an exhaust manifold 718, an EGR cooler 712, ahigh pressure variable geometry turbocharger 724, a low pressure turbine722 and an exhaust outlet 730. Only one cylinder 742 is shown forclarity but the engine would include multiple cylinders 742 and theoutput from the multiple cylinders 742 would be input to the exhaustmanifold 718. The vanes of the variable geometry turbocharger 724 can beadjusted to control the EGR flow and fresh air flow.

FIG. 8 illustrates an exemplary turbocharger vane position controlmodel. At block 810, the desired EGR rich mass flow and desired intakemanifold pressure rich setpoint are calculated. At block 814, theminimum exhaust manifold pressure to achieve the desired EGR rich massflow is calculated. An integral controller can be used to regulate theintake manifold pressure at block 818 by adjusting the exhaust manifoldpressure. At block 822, the desired exhaust manifold pressure is limitedto the minimum pressure calculated for EGR mass flow. At block 826, theturbine mass flow to achieve that pressure is calculated. Finally, atblock 830, the vane position is calculated.

The feed forward term for the turbine mass follow can be calculated bysubtracting the EGR mass flow from the engine out mass flow.{dot over (m)} _(trb_ff) ={dot over (m)} _(eng_out) −{dot over (m)}_(egr)  (65)This turbine flow will tend to keep the exhaust manifold pressureconstant. The closed loop term is used to make changes in pressure.

The desired EGR mass flow at block 810 can be calculated by firstcalculating the desired exhaust manifold pressure. The desired air massand diluent mass in-cylinder can be calculated as:

$\begin{matrix}{m_{{a\_{cyl}}{{\_ r}{ich}}{\_ des}} = \frac{m_{f\_{des}}}{F/A_{{rich}\_{des}}}} & (66) \\{m_{{d\_{cyl}}{{\_ r}{ich}}{\_ des}} = {m_{{a\_{cyl}}{{\_ r}{ich}}{\_ des}} \cdot {D/A_{des}}}} & (67)\end{matrix}$The desired mass of air, diluent, and total mass entering the cylindercan be calculated using the desired in-cylinder values and the residualvaluesm _(a_cyl_in_rich_des) =m _(a_cyl_rich_des) −m _(a_res)  (68)m _(d_cyl_in_rich_des) =m _(d_cyl_rich_des) −m _(d_res)  (69)m _(cyl_in_rich_des) =m _(a_cyl_in_rich_des) +m_(d_cyl_in_rich_des)  (70)The desired intake manifold rich diluent mass fraction can be calculatedby dividing the desired cylinder in rich diluent mass by the desiredcylinder in rich mass.

$\begin{matrix}{\chi_{{d\_{im}}{\_{rich}}{\_{des}}} = \frac{m_{{d\_{cyl}}{\_ in}{{\_ r}{ich}}{\_ des}}}{m_{{cyl\_ in}{{\_ r}{ich}}{\_ des}}}} & (71)\end{matrix}$The desired intake manifold rich diluent mass fraction can be related tothe desired intake manifold rich EGR mass fraction by:χ_(d_im_rich_des)=χ_(d_em)·χ_(egr_im_rich_des)+χ_(d_cac)·(1−χ_(egr_im_rich_des))  (72)The mass fraction of diluent in the charge air cooler is used in thiscalculation to account for water vapor that may enter with the freshair. Water vapor is considered a diluent. Equation (72) can berearranged to solve for the desired intake manifold rich EGR massfraction as:

$\begin{matrix}{\chi_{{{egr}\_{im}}{\_{rich}}{\_{des}}} = \frac{\chi_{{d\_{im}}{\_{rich}}{\_{des}}} - \chi_{d\_{cac}}}{\chi_{d{\_ e}m} - \chi_{d\_{cac}}}} & (73)\end{matrix}$The desired engine in rich mass flow is calculated using the desiredcylinder in rich mass, number of cylinders, and engine speed as:

$\begin{matrix}{{\overset{.}{m}}_{{{eng}\_{in}}{\_{rich}}{\_{des}}} = {m_{{{cyl}\_{in}}{\_{rich}}{\_{des}}} \cdot n_{cyl} \cdot {N_{e}({rpm})} \cdot ( \frac{\min}{60\mspace{14mu}\sec} ) \cdot ( \frac{cycle}{2\mspace{14mu}{rev}} )}} & (74)\end{matrix}$The desired rich EGR mass flow can then be calculated by multiplying thedesired intake manifold rich EGR mass fraction by the desired engine inrich mass flow as:{dot over (m)} _(egr_rich_des)=χ_(egr_im_rich_des) ·{dot over (m)}_(eng_in_rich_des)  (75)

At block 814, the minimum exhaust manifold pressure is found thatprovides the desired EGR mass flow. When the desired EGR valve positionu_(egr_des) is less than the maximum u_(egr_max), then there issufficient exhaust manifold pressure for EGR flow control with the EGRvalve. Under this condition, the minimum exhaust manifold pressure limitcan be calculated using the desired EGR rich mass flow and desiredintake manifold pressure rich setpoints. The exhaust pressurecalculation can take the desired rich intake manifold pressure and addthe calculated pressure drop across the EGR cooler and EGR valve (withthe EGR valve at its maximum position).

When the EGR valve is at the maximum position u_(egr_max), then the EGRvalve is not controlling the flow and the flow can be controlled byregulating the exhaust pressure. Under this condition, the desired EGRvalve mass flow from the EGR valve control calculation can be used withthe current intake manifold pressure estimate to calculate the requiredexhaust manifold pressure.

An exemplary calculation for the minimum exhaust manifold pressure underthe two conditions described above is:If u _(egr_des) <u _(egr_max)P _(em_min_for_egr)=ExhManifPressMinForEGR({dot over (m)}_(egr_rich_res) ,P _(im_rich_des))ElseP _(em_min_for_egr)=ExhManifPressMinForEGR({dot over (m)} _(egr_des) ,P_(im))  (76)

The desired intake manifold pressure can be determined at block 810 bycomputing the pressure needed to achieve the desired engine in mass flowwhich can be found using the ideal gas law and the volumetric efficiencyas:

$\begin{matrix}{P_{{{im}\_{rich}}{\_{des}}} = \frac{n_{cyl} \cdot m_{{{cyl}\_{in}}{\_{rich}}{\_{des}}} \cdot R \cdot T_{im}}{V_{d} \cdot {VE}}} & (77)\end{matrix}$This equation is related to the speed-density calculation but does notaccount for speed because the mass per cylinder is used instead of amass flow. The desired intake manifold rich pressure may be limited to acertain range for component protection and engine performance. Lowpressures are sometimes avoided to prevent high oil consumption and poorcombustion quality.

At block 818, the error in intake manifold rich pressure can becalculated as the difference between the desired intake manifold richpressure and the estimated intake manifold pressure by:P _(im_rich_error) =P _(im_rich_des) −P _(im)  (78)This pressure error can be multiplied by the integral gain and added tothe previous desired exhaust manifold pressure to determine theunlimited desired exhaust manifold pressure before the pressure limitsare applied as:P _(em_des_unlimited) =K _(i_im) ·P _(im_rich_error) +P_(em_des_previous)  (79)

At block 822, the unlimited desired exhaust manifold pressure may belimited by minimum pressure limits to the minimum exhaust manifoldpressure for EGR from equation (76), and may also have a calibrate-ableminimum limit for engine performance:P _(em_des_min_limited)=maximum(P _(em_des_unlimited) ,P_(em_min_for_egr) ,P _(em_min_for_perf))  (79A)Keeping the exhaust manifold pressure higher than necessary at low loadcan improve the transient response of the engine because theturbocharger speed is higher before the load is applied. Thisimprovement in transient response performance comes with a reduction infuel economy because the higher exhaust manifold pressure creates morepumping work for the engine.

The desired exhaust manifold pressure may be reduced when it isdetermined that controlling to a lower exhaust manifold pressureproduces more turbine power. As the exhaust manifold pressure increases,the turbine vanes move in the closing direction to achieve the desiredturbine mass flow. As the turbine vanes move in the closing directionthe angle at which the gasses enter the turbine changes. For a given setof operating conditions and turbocharger speed there will be an optimumturbine vane position that maximizes turbine power. Closing the vanesmore than what is optimum for maximum turbine power increase the exhaustmanifold pressure, increases engine pumping work, reduces fuel economy,and reduces turbine power. When it is determined that operating at alower exhaust manifold pressure produces more turbine power as describedbelow, the desired exhaust manifold pressure can be calculated by takingthe previous desired exhaust manifold pressure and subtracting a smallcalibrate-able value. Since this reduction in pressure occurs at eachtime step in the software the desired exhaust manifold pressure isreduced over time in the form of a ramp.

If Maximum_Turbine_PowerExceededP _(em_des_trb_pwr_limited) =P _(em_des_previous) −ΔP_(em_ramp_trb_pwr_exceeded)ElseP _(em_des_trb_pwr_limited) =P _(em_des_min_limited)  (79B)

The desired exhaust manifold pressure may also be constrained by amaximum engine delta pressure limits for engine protection. Engine deltapressure is the difference between the exhaust manifold pressure andintake manifold pressure. If the engine delta pressure becomes toolarge, the exhaust pressure can push the exhaust valve open during theintake stroke of the engine which is called valve float. A maximumexhaust manifold pressure limit for engine delta pressure protection canbe calculated as:P _(em_max_for_eng_dp) =P _(im) +ΔP _(eng_max)  (79C)The desired exhaust manifold pressure may also be constrained by amaximum pressure limit for component protection (to protect turbochargeseals, exhaust manifold gaskets, etc.). This pressure limit can becalculated using the ambient (barometric) pressure and the gage pressurelimit for the exhaust manifold components as:P _(em_max_for_comp) =P _(amb) +P _(em_max_comp_gage)  (79C)The desired exhaust manifold pressure can be calculated by taking theminimum of the turbine power limited desired exhaust manifold pressure,the maximum pressure for engine delta pressure protection, and thepressure limit for component protection:P _(em_des)=minimum(P _(em_des_trb_pwr_limited) ,P _(em_max_for_eng_dp),P _(em_max_for_comp))  (79D)In addition to the magnitude limits described here, rate limits can alsobe applied to prevent sudden changes to the desired exhaust manifoldpressure that would cause large transient changes to the desired turbinemass flow and large transient changes to the desired turbine vaneposition.

The closed loop turbine mass flow can be calculated by first calculatingthe exhaust manifold pressure error as:P _(em_error) =P _(em_des) −P _(em)  (80)where P_(em) is the exhaust manifold pressure estimate from theobserver. The estimate can be used instead of the measured value sincethe measured value may have lag due to the sensor response time and anyfiltering that may be used to reduce signal noise, and the estimatedvalue may continue to be used if a sensor fails. Alternatively, pressuresensors can respond fast compared to the air system response so usingthe measured pressure instead of the estimate can be done with similarresults. The exhaust manifold pressure error can be multiplied by theproportional gain to determine the closed loop term by:{dot over (m)} _(trb_cl) =−K _(p_em) ·P _(em_error)  (81)The negative sign is used because turbine flow has to go down toincrease exhaust manifold pressure. This allows a positive gain to beused in calculating the closed loop turbine mass flow. The desiredturbine mass flow is the sum of the feed forward term and the closedloop term:{dot over (m)} _(trb_des) ={dot over (m)} _(trb_ff) +{dot over (m)}_(trb_cl)  (82)

The control method described above and illustrated in FIG. 8 is intendedfor use when the desired lean F/A setpoint is less than the desired richF/A setpoint. This results in the air throttle attempting to maintain ahigher intake manifold pressure than the turbine vane position control.This separation can create a control band in which both the air throttleand turbine vanes move in the opening direction. Under steady stateconditions the air throttle typically moves to the maximum position,which minimizes the pumping work across the air throttle.

When the desired lean F/A setpoint is greater than or equal; to thedesired rich F/A setpoint, a different control method can be used toprevent the air throttle and turbine vane controls from conflicting.Without the intake manifold pressure setpoint separation, the airthrottle control and turbine vane control may oscillate or the airthrottle may close completely. This problem can be overcome by using theturbine vanes to control the charge air cooler pressure instead of usingthe turbine vanes to control the intake manifold pressure.

In this situation, the desired air throttle rich mass flow and thedesired charge air cooler pressure can be calculated first. The desiredair throttle rich mass flow can be calculated using the desired intakemanifold rich EGR mass fraction and the desired engine in rich mass flowas:{dot over (m)} _(at_rich_des)=(1−χ_(egr_im_rich_des))·m_(eng_in_rich_des)  (83)

The desired charge air cooler rich pressure can be estimated by thedesired intake manifold rich pressure and the pressure drop across theair throttle and charge air cooler. This calculation can be shown as afunction by:P _(cac_rich_des)=DesCACPress(u _(at_max) ,{dot over (m)} _(at_rich_des),P _(im_rich_des) ,T _(caci) ,T _(caco))  (84)This function has two parts. First, equation (12) can be used to findthe desired charge air cooler outlet pressure that provides the desiredmass flow through the air throttle. Then equation (21) can be used tofind the charge air cooler inlet pressure that is sufficient to overcomethe pressure drop across the charge air cooler. The maximum air throttleposition u_(at_max) can be used since it minimizes the pumping workacross the air throttle. These equations can be solved by an iterativesolution method to determine the desired charge air cooler richpressure.

The charge air cooler pressure rich error is the difference between thedesired charge air cooler rich pressure and the estimated charge aircooler pressure:P _(cac_rich_error) =P _(cac_rich_des) −P _(cac)  (85)This error can be multiplied by the integral gain and added to theprevious desired exhaust manifold pressure to determine the desiredexhaust manifold pressure as:P _(em_des) =K _(i_cac) ·P _(cac_rich_error) +P _(em_des_previous)  (86)The desired exhaust manifold pressure can then be limited to the minimumvalue calculated by equation (76), and it may also be limited to acertain range for component protection and engine performance. Thedesired exhaust manifold pressure can then be used to calculate thedesired turbine mass flow as shown by equations (80) through (82).

The desired turbine vane position u_(vgt_des) can be calculated usingthe desired turbine mass flow {dot over (m)}_(trb_des) and the stateinformation for the observer model. The state information can include,for example, exhaust manifold pressure, exhaust manifold temperature,exhaust back pressure, high pressure turbocharger speed, and lowpressure turbocharger speed.

The interstage pressure between the high and low pressure turbines 724,722 can also be determined in finding the vane position u_(vgt_des) thatprovides the desired turbine mass flow {dot over (m)}_(trb_des). Theturbine interstage pressure and desired vane position can be found for agiven turbine mass flow using a two dimensional binary search, anexample of which is shown in FIG. 9.

At block 910, the desired turbine mass flow {dot over (m)}_(trb_des)from equation (82) is input. At block 912, the exhaust manifold pressureand temperature, the exhaust back pressure, and the speeds of the lowand high pressure turbines 724, 722 are input. At block 914, the currentdesired vane position is set to the desired vane position from thecalculation that was performed at the previous time step, the turbinedelta pressure is set to the difference between the exhaust manifoldpressure and the exhaust back pressure, and a search index and vaneadjustment value are initialized. Block 916 is an outer loop check thatallows j_(max) iterations of the outer loop. Block 920 is an inner loopthat performs j_(max) iterations adjusting the turbine interstagepressure to find a resulting turbine interstage pressure for the currentdesired vane position that provides the same or closest to the same massflow through each of the turbines 724, 722. This step is similar to thesearch described herein with reference to the observer model but in thiscase the model is being used to evaluate a possible vane position ratherthan the final desired vane position. At block 930, the outer loop takesthe mass flows through each of the turbines 724, 722, and calculates anaverage turbine mass flow by averaging the high and low pressure turbinemass flows. At block 930, the outer loop also fine tunes the turbineposition adjustment step. At block 934, the average turbine mass flow iscompared with the desired turbine mass flow. If the average turbine massflow is too high, then at block 936 the current desired vane position isadjusted in the closing direction (increasing u_(vgt_des)). Otherwise,at block 938 the current desired vane position is adjusted in theopening direction (decreasing u_(vgt_des)). Control returns to block916, and the process is repeated as part of the binary search. After allthe iterations of the outer loop are complete, at block 950 the finaldesired vane position u_(vgt_des) that provides the desired turbine massflow {dot over (m)}_(trb_des) is output. This process can be repeatedwith percentage adjustments to the vane position of, for example, 4, 2,1, 0.5 and 0.25 percent.

The modeled mass flow through the high and low pressure turbines 724,722 in the search for the desired vane position can each be correctedusing the turbine mass flow correction factor shown in equation (30).This allows the turbine vane position to be determined with modeled flowcorrection so that the desired turbine mass flow is achieved.

The turbine interstage temperature can be assumed to be equal to thehigh pressure turbine outlet temperature, which can be calculated usingthe turbine efficiency, pressure ratio across the high pressure turbine,and the exhaust manifold temperature. The efficiency can be determinedusing tables with inputs of corrected turbocharger speed and pressureratio. The turbocharger speed can be adjusted using a correction factorto compensate for operating with exhaust manifold temperatures that aredifferent than the reference temperature used when creating theefficiency table. Efficiency tables can be generated for different vanepositions and interpolation can be used when the vane position isbetween two of the provided tables. The turbine interstage pressure andtemperature can be used to calculate the low pressure turbine mass flow.

For a given turbine mass flow rate and turbocharger speed there will bean optimum turbine vane position that maximizes turbine power. This vaneposition will provide the best combination of flow velocity and flowangle for the exhaust gas as it enters the turbine blades. Opening thevanes more than the maximum turbine power point causes lower velocity,less optimum flow angle, and less turbine power but it also reduces theexhaust manifold pressure which tends to reduce engine pumping work. Fornormal control when the maximum turbine power is not needed the vanesare typically positioned more open than the maximum turbine power pointbecause this reduces the engine pumping work.

Closing the vanes more than the maximum turbine power point causeshigher velocity but the flow angle is less optimum, there is lessturbine power, and the exhausts manifold pressure is higher. This regionof operation is typically avoided. One exception is when there is a needto raise the exhaust temperature for after-treatment system control. Inthis case, the increase in pumping work is beneficial because it putsadditional load on the engine. The extra load on the engine requiresmore fuel to be consumed, raising the exhaust temperature.

For normal control it is desirable to have the turbine vanes between themaximum turbine power position and wide open. When calculating thedesired vane position for a desired turbine mass flow, exhaust manifoldpressure, and turbocharger speed as described above, a check can also beperformed to see if the maximum turbine power has been exceeded. Whenthis happens it means the desired exhaust manifold pressure is too high.

This maximum turbine power check can be performed after the vaneposition calculation described above to see if controlling to a slightlylower exhaust manifold pressure (5 kPa lower for example) would providemore turbine power. If the turbine power is higher with reduced exhaustmanifold pressure that means the exhaust manifold pressure for maximumturbine power has been exceeded. When the maximum turbine power isexceeded, the desired exhaust manifold pressure can be reduced at acalibrate-able rate until the maximum turbine power is no longerexceeded, as shown by equation (79B).

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, such illustration and description isto be considered as exemplary and not restrictive in character, it beingunderstood that illustrative embodiment(s) have been shown and describedand that all changes and modifications that come within the spirit ofthe disclosure are desired to be protected. It will be noted thatalternative embodiments of the present disclosure may not include all ofthe features described yet still benefit from at least some of theadvantages of such features. Those of ordinary skill in the art mayreadily devise their own implementations that incorporate one or more ofthe features of the present disclosure and fall within the spirit andscope of the present invention as defined by the appended claims.

We claim:
 1. A fresh air and exhaust gas control method for an enginehaving an air system with an air throttle, an exhaust gas recirculation(EGR) valve, an intake manifold, a charge air cooler, an exhaustmanifold, and a turbocharger, the control method comprising: monitoringparameters of the engine in an operational state using a plurality ofsensors, the plurality of sensors including an intake manifold pressuresensor and an exhaust manifold pressure sensor; generating engine stateestimates using an engine observer model, the engine observer modelrepresenting the intake manifold volume, the exhaust manifold volume,and the charge air cooler volume, the engine state estimates based onthe monitored engine parameters from the plurality of sensors;generating a turbocharger rotational speed estimate using a turbochargermodel; calculating a fresh air flow correction factor based on thedifferences between the measured engine states and the engine stateestimates and inputting the fresh air flow correction factor to theengine observer model; determining a desired air throttle position and adesired EGR valve position based on setpoint commands, the monitoredengine parameters, the fresh air flow correction factor, the enginestate estimates, and the turbocharger rotational speed estimate;adjusting the air throttle based on the desired air throttle position;and adjusting the EGR valve based on the desired EGR valve position. 2.The fresh air and exhaust gas control method of claim 1, wherein theplurality of sensors excludes a mass air flow sensor.
 3. The fresh airand exhaust gas control method of claim 1, wherein the turbochargercomprises a compressor and a turbine, and wherein the turbochargerrotational speed estimate is used to estimate a flow through thecompressor.
 4. The fresh air and exhaust gas control method of claim 3,wherein the flow through the compressor is used to estimate an inletpressure to the charge air cooler.
 5. The fresh air and exhaust gascontrol method of claim 3, wherein generating engine state estimatesusing an engine observer model includes generating an estimate of intakemanifold pressure.
 6. The fresh air and exhaust gas control method claimof 1, wherein generating engine state estimates using an engine observermodel includes generating estimates of the inlet and outlet flows ofeach of the intake manifold volume, exhaust manifold volume, and chargeair cooler volume.
 7. The fresh air and exhaust gas control method ofclaim 1, wherein the turbocharger comprises a compressor and a turbine,the control method further comprising: calculating a turbine intakecorrection factor based on the differences between the measured enginestates and the engine state estimates and inputting the turbine intakecorrection factor to the engine observer model, wherein the determininga desired air throttle position and a desired EGR valve positionincludes determining a desired air throttle position and a desired EGRvalve position based on the turbine intake correction factor.
 8. Thefresh air and exhaust gas control method of claim 1, further comprising:calculating an incremental EGR correction factor based on thedifferences between the measured engine states and the engine stateestimates and inputting the incremental EGR correction factor to theengine observer model, wherein the determining a desired air throttleposition and a desired EGR valve position includes determining a desiredair throttle position and a desired EGR valve position based on theincremental EGR correction factor.
 9. The fresh air and exhaust gascontrol method of claim 8, wherein the fresh air flow correction factorand incremental EGR correction factor are applied to a feed forwarddetermination of the desired air throttle position and the desired EGRvalve position.
 10. The fresh air and exhaust gas control method ofclaim 1, wherein the turbocharger is a two-stage turbocharger having afirst and a second compressor in a series arrangement, whereingenerating engine state estimates includes generating an estimate of aninterstage pressure between the first compressor and the secondcompressor.
 11. The fresh air and exhaust gas control method of claim10, wherein generating the estimate of an interstage pressure betweenthe first compressor and the second compressor includes estimating apressure between the first compressor and the second compressor at whichthe mass flow through the first compressor equals the mass flow throughthe second compressor.
 12. The fresh air and exhaust gas control methodof claim 1, further comprising: calculating a desired intake manifolddiluent mass fraction based on a desired diluent-to-air ratio setpoint,an engine speed and a number of cylinders in the engine; calculating adesired EGR mass flow based on the desired intake manifold diluent massfraction; calculating the desired EGR valve position based on thedesired EGR mass flow; calculating a desired intake manifold pressurebased on the desired diluent-to-air ratio setpoint and the number ofcylinders in the engine; calculating a desired air throttle mass flowbased on the desired intake manifold pressure; and calculating thedesired air throttle position based on the desired air throttle massflow.
 13. A fresh air and exhaust gas control method for an enginehaving an air system with an air throttle, an exhaust gas recirculation(EGR) valve, an intake manifold, a charge air cooler, an exhaustmanifold, and a turbocharger including a turbine, the control methodcomprising: monitoring parameters of the engine in an operational stateusing a plurality of sensors, the plurality of sensors including anintake manifold pressure sensor and an exhaust manifold pressure sensor;generating engine state estimates using an engine observer model, theengine observer model representing the intake manifold volume, theexhaust manifold volume, and the charge air cooler volume, the enginestate estimates based on the monitored engine parameters from theplurality of sensors; generating a turbocharger rotational speedestimate using a turbocharger model; calculating a turbine intakecorrection factor based on the differences between measured enginestates and the engine state estimates and inputting the turbine intakecorrection factor to the engine observer model; determining a desiredair throttle position and a desired EGR valve position based on setpointcommands, the monitored engine parameters, the turbine intake correctionfactor, the engine state estimates, and the turbocharger rotationalspeed estimate; adjusting the air throttle based on the desired airthrottle position; and adjusting the EGR valve based on the desired EGRvalve position.
 14. The fresh air and exhaust gas control method ofclaim 13, wherein the plurality of sensors excludes a mass air flowsensor.
 15. The fresh air and exhaust gas control method of claim 13,further comprising: calculating an incremental EGR correction factorbased on the differences between the measured engine states and theengine state estimates and inputting the incremental EGR correctionfactor to the engine observer model, wherein the determining a desiredair throttle position and a desired EGR valve position includesdetermining a desired air throttle position and a desired EGR valveposition based on the incremental EGR correction factor.